This invention pertains generally to the field of electro-discharge machining and to micro-electromechanical devices and processes for producing such devices.
Micro-electro-discharge machining (micro-EDM) is a microfabrication technique that is well suited to cutting electrically conductive materials such as steel, graphite, silicon and magnetic materials. See, e.g., D. Reynaerts, et al., xe2x80x9cIntegrating Electro-Discharge Machining and Photolithography: Work in Progress,xe2x80x9d J. of Micromechanics and Microengineering, Vol. 10, No. 2, June, 2000, pp. 189-195; Y. Honma, et al., xe2x80x9cMicro-Machining of Magnetic Metal Film Using Electro-Discharge Technique,xe2x80x9d Advances in Information Storage Systems, Vol. 10, 1999, pp. 383-399; C. A. Grimes, et al., xe2x80x9cMagnetoelastic Microsensors for Environmental Monitoring,xe2x80x9d Tech. Dig., IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""01), Interlaken, Switzerland, January, 2001, pp. 278-281. Micro-EDM involves the sequential discharge of electrical pulses between a microscopic electrode and the workpiece while both are immersed in a dielectric oil. See, generally, T. Masaki, et al., xe2x80x9cMicro Electro-Discharge Machining and its Applications,xe2x80x9d Proc., IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""90), Napa Valley, Calif., February, 1990, pp. 21-26. The pulse discharge timing is controlled by a simple resistor-capacitor (RC) circuit. In conventional micro-EDM, the electrode is a cylindrical metal element from 5 to 300 xcexcm in diameter. Although micro-EDM has been used commercially for applications such as ink-jet nozzle fabrication, the traditional process is limited in throughput because it is a serial process. The use of a single electrode limits not only the throughput, but also precision, because the electrodes themselves are individually shaped by using a micro-EDM technique-wire electrode-discharge grinding (WEDG)xe2x80x94and variation may occur in the electrode shape. See, e.g., T. Masuzawa, et al., xe2x80x9cWire Electro-Discharge Grinding for Micro-Machining,xe2x80x9d Ann. CIRP, Vol. 34, 1985, pp. 431-434.
To address the throughput and material issues that limit conventional micro-EDM, batch mode micro-EDM has been developed using LIGA-fabricated electrodes. The LIGA process uses x-ray lithography to form high aspect ratio molds for electroplated structures. For a general discussion of the LIGA process, see W. Ehrfeld, et al., xe2x80x9cLIGA Process: Sensor Construction Techniques via X-Ray Lithography,xe2x80x9d Tech. Dig., IEEE Intl. Conf. on Solid-State Sensors and Actuators Workshop (Hilton Head ""88), June, 1988, pp. 1-4. Electroplated copper electrodes formed using the LIGA process have been shown to provide acceptable wear resistance. K. Takahata, et al., xe2x80x9cA Novel Micro Electro-Discharge Machining Method Using Electrodes Fabricated by the LIGA Process,xe2x80x9d Tech. Dig., IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""99), Orlando, Fla., January, 1999, pp. 238-243. Parallel machining to provide perforations in stainless steel by using 3xc3x974 arrayed electrodes with 100 xcexcm diameter and 500 xcexcm pitch was also demonstrated. Sequential application with electrode arrays has also been utilized to produce a 1-mm long WC-Co super-hard alloy mechanical processing tool. K. Takahata, et al., xe2x80x9cHigh-Aspect-Ratio WC-Co Microstructure Produced by the Combination of LIGA and Micro-EDM,xe2x80x9d Microsystem Technologies, Vol. 6, No. 5, August, 2000, pp. 175-178. LIGA fabricated EDM electrode arrays have been developed to achieve parallelism and increase throughput. K. Takahata, et al., xe2x80x9cBatch Mode Micro-EDM for High-Density and High-Throughput Micromachining,xe2x80x9d IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""01), 2001, pp. 72-75; Ken""ichi Takahata, et al., xe2x80x9cParallel Discharge with Partitioned Electrode Arrays for Accelerated Batch Mode Micro-EDM,xe2x80x9d Transducers ""01, Germany, Jun. 4, 2001.
In accordance with the invention, parallel batch micromachining is carried out utilizing a semiconductor array electrode that may be formed by conventional semiconductor processing techniques at low cost. The semiconductor array electrode includes a semiconductor substrate having a face surface and multiple semiconductor electrode protrusions extending from the face surface formed integrally with the substrate semiconductor. The substrate and protrusions are doped to be electrically conductive, and may be coated with a metal to increase conductivity and wear resistance. The semiconductor array electrode may be formed utilizing semiconductor processing techniques by masking a wafer of semiconductor, such as crystalline silicon, in a pattern corresponding to the electrode protrusions to be formed. An anisotropic etchant is then applied to the masked wafer to anisotropically etch the wafer in the regions not covered by the mask. The mask is then removed to leave the protrusions extending above the etched regions of the wafer.
In use, the array electrode is positioned adjacent to a workpiece to be machined, with dielectric fluid between the array electrode and the workpiece. A voltage is then applied from a voltage source between the workpiece and the array electrode to charge a capacitor conducted between the workpiece and the electrode. The charge on the capacitor builds until it reaches a sufficient level to provide a discharge of current between the electrode protrusions and the workpiece to micromachine the workpiece.
The present invention is well suited to carry out micromachining on materials, such as brass, which are difficult to etch using lithographic processes. Although the semiconductor electrode is consumed during the micromachining process and is disposed after a single use, a large semiconductor wafer (particularly silicon) can be lithographically patterned over the surface of the wafer and can be applied to micromachine many parts in a workpiece simultaneously, providing highly efficient and low cost micromachining of multiple parts.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.